1. Field of the Invention
The present invention relates to a method for forming a predetermined thin film on the surface of a to-be-processed material.
2. Description of the Related Art
Various methods have been known as the technique for forming a predetermined thin film on the surface of a to-be-processed material. In the process for manufacturing semiconductor devices, for example, a silicon-based thin film, such as silicon nitride or silicon oxide, is formed on a to-be-processed material, such as a semiconductor wafer, many times in view of the necessity to form many microminiaturized elements. In this case, in order to not only achieve a higher quantity-production efficiency than a certain production level but also to achieve stabilized characteristics, that is, desired electrical properties, of semiconductor devices (final products), it is required that, during film formation processing, films be uniformly formed, as improved uniform thickness films, across the surfaces of wafers.
The film forming processing is carried out by a processing apparatus, such as a pressure-reduced CVD apparatus. For example, dichlorosilane is used as a processing gas for the formation of a silicon nitride film and a TEOS (tetraethoxyorthosilane), etc., is used as a processing gas for the formation of a silicon oxide film. In the case where films are formed on wafers by a processing apparatus equipped with a double tube-type upright processing container, a processing gas is introduced from below into an inner tube of a processing container with many wafers placed one above another over a wafer boat. The processing gas flows up from below into the inner tube with a wafer boat placed in the inner tube. As a result, a reaction product is deposited on the surface of the wafer. The processing gas, after reaching the upper area of the processing container, downwardly flows through a space between the inner tube and an outer tube and is exhausted past a gas exhaust system out of the processing container.
Forming a silicon nitride film (Si.sub.3 N.sub.4) on the surface of the wafer will be explained below by way of example.
In the film forming processing, use is made of two kinds of processing gases, that is, dichlorosilane (SiH.sub.2 Cl.sub.2) and ammonia (NH.sub.3). Through a reaction of dichlorosilane and ammonia given by EQU 3SiH.sub.2 Cl2+4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6HCl+6H.sub.2 (b 1)
a silicon nitride film (Si.sub.3 N.sub.4) is formed on the surface of the wafer.
The sequence of the film forming processing is shown in FIG. 5. As Shown in FIG. 5, for example, ammonia is first flowed in a predetermined quantity, for example, at 1000 SCCM and, after a lapse of 5 minutes, dichlorosilane is flowed in a predetermined quantity, for example, at 100 SCCM for film forming processing to be carried out. During film forming processing, both the gases are continuously flowed and, at the end of one cycle, the supplying of dichlorosilane is first stopped and then the supplying of ammonia is stopped after a lapse of, for example, 5 minutes. The reason why ammonia is necessarily supplied during the supply of dichlorosilane is because, if dichlorosilane alone is supplied in the absence of ammonia, a different film is deposited on the surface of the wafer through a different reaction.
In this type of method for forming a film through the continuous supply of the processing gas during one cycle, there occurs a gradual rise in a partial pressure of a reaction byproduct not consumed with a passage of a film formation time, that is, a partial pressure of HCl in particular in the case of the above chemical formula (1).
This prevents diffusion of dichlorosilane, a source gas, and adversely affects the uniformity of a film thickness over the surface of the wafer. The adverse effect of the reaction byproduct becomes more prominent toward the downstream (TOP) side than toward an upstream (BTM) side of the gas stream and more prominent toward the center than the marginal edge of the wafer. This is because a reaction in a processing gas is progressed toward the downstream (TOP) side. FIG. 6 shows the density distribution of a reaction byproduct (HCl) produced in a zone from the upstream (bottom) side toward the downstream (TOP) side of the gas stream and FIG. 7 shows the shape of films formed on the wafers at the top (TOP) and bottom (BTM) sides which have been affected by the byproduct. Since the reaction in the processing gas is progressed toward the downstream (TOP) side of the gas stream, the density of the byproduct HCl becomes higher from the bottom side toward the top side of the wafer boat as shown in FIG. 6. For this reason, as shown in FIG. 7, the uniformity of film thickness at the bottom (BTM) side across the surface of a wafer W1 is considerably better but the uniformity of film thickness at the top side across the surface of a wafer W3 is adversely affected because the adverse effect from the byproduct becomes prominent toward the top (downstream) side. This phenomenon markedly appears as the time over which a film is deposited on the wafer becomes longer, that is, the thickness of a film to be deposited becomes greater.
In an experiment for forming, for example, an SiN thin film on the surface of a wafer, a 600 .ANG.-thick and a 1800 .ANG.-thick film were formed on the wafer surface and comparisons were made against data obtained three times for each case. For the 600 .ANG.-thick film, the uniformity of film thickness on the surface of a wafer W3 at a top side was 2.06%, 2.12% and 2.07%, respectively, an average being 2.08%. For the 1800 .ANG.-thick film, on the other hand, the uniformity of film thickness on the surface of a wafer W3 at a TOP side was 2.75%, 2.76% and 2.75%, an average being 2.75%. From this it has been found that a longer film formation time adversely affects the uniformity of film thickness on the wafer surface. It has also been found that even in the case of a 600 .ANG.-thick film thickness, the uniformity of the film thickness over the surface of a wafer W3 at the TOP side was .+-.2.0% whereas the uniformity of the film thickness over the surface of a wafer W1 at the BTM side was .+-.1.9% and that the uniformity of the film thickness was adversely affected more toward the TOP side. In the case where a 1800 .ANG.-thick film was formed on the wafer surface, the uniformity of the film thickness of a wafer W3 at the TOP side was .+-.2.7% whereas the uniformity of the film thickness of a wafer W1 at the BTM side was .+-.2.1%. The uniformity of the film thickness at the TOP side of the wafer is adversely affected but, compared with the 600 .ANG.-thick film, a difference in the uniformity of the film thickness at the TOP and BTM sides is great.
Various methods may be conceived as a means for computing the uniformity of the film thickness. For example, it may be possible to adopt a method for obtaining a film thickness uniformity value by setting several measuring points on the wafer's film surface and dividing a difference of maximum and minimum ones of these measuring points by double an average thickness at the measuring points. It may be possible to represent the film thickness uniformity value as a standard deviation in the case where a larger number of measuring points are set on the wafer's film surface. In either case, the smaller the film thickness uniformity value the better.
As shown in FIG. 8, at the start of film formation, a film thickness on the wafer surface is greater at the TOP side than at the BTM side, but, as the film formation time progresses, the thickness of the film is reversed and becomes greater at the BTM side than at the TOP side, thus the film thickness becomes nonuniform among the wafer surfaces. The reason for such reversion is because a source gas is adequately diffused at the TOP side in particular due to a lower density of a reaction byproduct at the start of film formation and an amount of film deposition is increased at the TOP side, but, with further progress of film formation, the density of the byproduct becomes greater at the TOP side in particular and the reaction is suppressed at the TOP side.
In order to maintain the uniformity of the film thickness among the wafer surfaces, a temperature gradient at the processing temperature is provided between the BTM side and the TOP side for a target film thickness of, for example, 1000 .ANG. so that the film thickness lines of the bottom and top sides cross as shown in FIG. 8 when the film thickness reaches a level 1000 .ANG.. FIG. 9 shows one form of the temperature gradient at that time. As shown in FIG. 9, when, for example, the film formation temperature is set to 780.degree. C. at the center (CTR) side, the film formation temperature is set to be 770.degree. C. at the bottom side and 800.degree. C. at the top side so as to achieve a uniform film thickness among the wafer surfaces.
When the temperature difference is provided between the bottom side and the top side however a nonuniform film is obtained due to the presence of a different film quality across the wafer surface and, for example, a different etching rate, etc., occurs, thus posing a problem. If nonuniformity occur in the quality of the film, a greater problem arises in the case where, in view of a recent large increase of an integration density, a 64 Mbit D-RAM element for instance is to be manufactured.
In order to solve the nonuniformity of the film thickness among the wafer surfaces, a special gas injection tube, that is, an injector, extending in the length direction of the wafer boat is used in which case a gas is injected from many gas injection holes so as to achieve a uniform gas stream in a zone between the bottom side and the top side. In spite of this means, that structure per se becomes very complex and higher in manufacturing costs add, moreover, the gas injection hole diameters are gradually varied as a result of repeated uses and wastes, thus causing lowered reproducibility.